Wellbores for the production of hydrocarbon fluid generally are provided with steel casings and/or liners to provide stability to the wellbore wall and to prevent undesired flow of fluid between the wellbore and the surrounding earth formation. In a conventional wellbore, the wellbore is drilled in sections whereby each section is drilled using a drill string that has to be lowered into the wellbore through a previously installed casing. In view thereof the wellbore and the subsequent casing sections decrease in diameter with depth. The production zone of the wellbore therefore has a relatively small diameter in comparison to the upper portion of the wellbore.
It has been proposed to drill a mono diameter wellbore whereby the casing or liner to be installed is radially expanded in the wellbore after lowering to the required depth. Subsequent wellbore sections therefore may be drilled at a diameter larger than in the conventional wellbore and, if each casing section is expanded to the same diameter as the previous section, the wellbore diameter may remain substantially constant with depth.
US 2006/0065403 A1 discloses an assembly for expanding a tubular member in a wellbore using an expanding cone that is pulled through the tubular member by a force multiplier suspended on drill string. The assembly is provided with a bottom packer below the expander cone. The bottom packer is set in a launcher section of the tubular member and seals the tubular member from the wellbore. A conventional packing setting mechanism is used to expand and set the packer in the launcher section. After expansion of the tubular member, the bottom packer is drilled out of the casing and the next portion of the wellbore is drilled to a next desired depth.
The conventional bottom packer of the known assembly is a massive device consisting of many components including a setting mechanism. Problems may therefore arise during drilling out the packer using a drill bit or milling tool. For example, there is an inherent risk that individual components of the packer come loose during drilling out and cause damage to the cutters of the drill bit or milling tool.
US patent application US2009/0266560 discloses a tubular expansion assembly provided with a bottom plug(118), which, as illustrated in FIGS. 3,4,6 and 9-12, is not radially expanded, but drilled out after expansion of the upper part of the tubular.
It is an object of the invention to provide an improved assembly for creating an expanded tubular element in a borehole extending into an earth formation wherein the bottom plug is also expanded.